This invention is directed to preparation of high molecular weight, crosslinked, water-soluble cationic polymers. Cationic polymers have been used extensively in water treatment, papermaking, mineral processing, petroleum recovery, fabrics, cosmetics and pharmaceuticals. Among the most important and extensively used cationic polymers are the quaternary ammonium polymers of diallyldialkyl ammonium compounds. It has been shown that the higher the molecular weight (MW) of the resulting cationic polymer, the more effective the polymer is as a flocculating agent.
Polymerization with added inorganic salts has been used to achieve high molecular weights. Polymerization of diallyldialkyl ammonium monomers is typically carried out in aqueous solution using a free radical initiator. Persulfate is commonly used as initiator for polymerization of the most commercially important diallydialkyl ammonium monomer, diallyldimethyl ammonium chloride (DADMAC).
U.S. Pat. No. 4,222,921 first discovered that the use of a diallylamine salt other than hydrohalide markedly speeds up the polymerization rate using ammonium persulfate (APS) as initiator. The conversion of monomer to polymer was substantially greater when the diallylamine salt polymerized was a salt of a strong acid (e.g., sulfuric acid) other than hydrohalide acids (e.g., hydrochloride acid). It was speculated that the halide ion acted as a chain transfer agent and a chain terminator.
Jaeger et al. (Macromol. Sci. Chem., A21(5):593, 1984) reported that persulfate could oxidize the chloride ion to produce chlorine radical which then terminated polymerization and decreased molecular weight. They obtained relatively high molecular weight poly-DADMAC using azo initiator instead of persulfate initiator.
U.S. Pat. No. 4,742,134 discloses that increased polymerization rate and molecular weight can be obtained using fluoride salts with persulfate initiator. Halide salts other than fluoride (e.g. NaCl) did not accelerate polymerization for increased molecular weight.
U.S. Pat. No. 5,248,744 discloses a method for making high molecular weight poly-DADMAC with an azo initiator.
U.S. Pat. No. 5,422,408 gave data of reduction potentials to show that persulfate is strong enough to oxidize chloride ions and bromide ions but not fluoride ions. Therefore, addition of chloride salts (e.g. NaCl) or bromide salts would not give increased molecular weight for polymerization of DADMAC using persulfate initiator. The reference disclosed a method to prepare polyDADMAC with increased molecular weight using an azo initiator in combination with added inorganic salts including NaCl salt.
U.S. Pat. No. 4,439,580 demonstrated that use of ammonium persulfate initiator with added NaCl salt in inverse emulsion polymerization also gave high molecular weight polyDADMAC. The narrow pH range (8.0 to 10.5) and added salts used in the inverse (water-in-oil) emulsion polymerization were claimed to be critical elements for the success of this invention.
U.S. Pat. No. 3,544,318 teaches that branched polyDADMAC works better than linear for electroconductive paper because the branched polymer imparts superior barrier properties to the electroconductive paper substrate, preventing solvent from diffusing into the paper.
U.S. Pat. No. 3,968,037 showed that cationic polymers obtained made by inverse (water-in-oil) emulsion polymerization with crosslinking and branching agents had surprisingly high effectiveness as flocculants and for the treatment of activated sewage sludge. The inventors used polyolefinic unsaturated compounds, such as tri- and tetra-allyl ammonium salts, or methylenebisacrylamide (MBA), as the crosslinking agents. They found that only ineffective products were obtained from solution polymerization containing a crosslinking agent.
Published European Pat. No. 264,710, however, claimed that highly branched water-soluble polyDADMAC made from solution polymerization also worked better as flocculants or defoaming agents for breaking oil-in-water emulsions. The branched polyDADMAC were made by adding 0.1 to 3.0 mole % of crosslinking comonomer such as methyltriallyl ammonium chloride (MTAAC) or triallylamine hydrochloride (TAAHCl) during progressive polymerization of DADMAC after monomer conversion had achieved at least 25% to 90%. A completely gelled product was obtained when the MTAAC was added all at once in the beginning.
U.S. Pat. No. 5,653,886 discloses the use of crosslinked DADMAC polymers as coagulants in suspensions of inorganic solids for mineral refuse slurry. The preferred high molecular weight crosslinked polyDADMAC for the application is prepared by copolymerization of DADMAC with acrylamide and triallylamine.
U.S. Pat. No. 5,989,382 uses a multifunctional (triallylamine) to make high molecular weight cross-linked poly-DADMAC, which can be used for pitch control in papermaking.
In studying interaction of cationic polyelectrolytes with counter anions, Ghimici et al (Journal of Polymer Science: Part B, Vol. 35, page 2571,1997) found that the cationic polyelectrolyte sample with more branching or crosslinking had stronger binding with anionic counter ions. The authors think that branching of the polycations creates regions with higher numbers of charged groups even at high dilution and consequently an increased number of counterions are associated to them. Similar explanation may be given to account for improved performance of branched or crosslinked polyDADMAC in coagulation and flocculation applications
U.S. Pat. No. 6,323,306 discloses a process for the preparation of a high molecular weight cationic polymer by crosslinking quaternary ammonium cationic base polymers with crosslinking agents capable of reacting with the amino functional groups of the cationic base polymer.
Peroxide compounds have been used to crosslink water-insoluble hydrocarbon polymers, without polar side-groups, in organic solvents or in melts. In 1914, it was discovered that dibenzoyl peroxide crosslinks rubber. The use of the more effective dialkyl peroxides to crosslink polymers started shortly after 1950. The crosslink is believed to be attained through free radicals formed by homolytic decomposition of the peroxide. The decomposition of peroxide produces radicals, which can abstract hydrogen from polymer chains. Coupling of the polymeric radicals leads to crosslinks for vulcanization. [Kirk-Othmer Concise Encyclopedia of Chemical Technology, published by John Wiley & Sons, Inc., 1985, page 1028]. However, the radical crosslinking is effective only for limited thermoplastic polymers. The radicals formed on polymer chains can lead to degradation as well as crosslinking. Polymer structure among other things decides whether crosslinking or degradation is likely to occur. It is known that treatment with peroxide in melts will cause polyethylene to crosslink but polypropylene to degrade. Radical crosslinking is not effective for butyl rubber or the like. An oil-soluble organic peroxide is generally used for radical crosslinking of hydrophobic water-insoluble polymers. A few water-soluble polymers containing polar side groups have been found to be crossslinkable by water-soluble radical initiators.
U.S. Pat. No. 3,168,500 discloses a method for making water-insoluble acrylamide polymers by crosslinking a water-soluble acrylamide polymer in the presence of a radical initiator. Peroxo compounds, such as potassium persulfate, hydrogen peroxide, or t-butyl hydroperoxide with or without a reducing agent when added in large quantities, usually greater than 10% based on the amount of the polymer solids, will gel polyacrylamide or copolyacrylamide solutions of great than 1% concentration. The initiator and the polymer are premixed and then the system is heated to a desired temperature until the polymer is converted into a water-insoluble gel. No reaction occurs with less than the minimum amount of about 5 to 10% of the initiator. The pH of the system during the process is not critical. The water-insoluble polymers of acrylamide can be used for the preparation of adhesives, soil stabilization or for treating paper, textiles, leather and the like. The polymers of acrylamide are non-ionic polymers or anionic copolymers. The patent does not describe the method for making water-soluble crosslinked polymers. It was demonstrated with examples that peroxides, such as hydrogen peroxide and t-butyl hydrogen peroxide, can effect crosslinking as well as persulfate compounds.
Published European Patent 208,945 discloses a method for making water absorbent acrylic polymers crosslinked by a peroxide radical initiator. Water-soluble acrylic acid polymers are premixed with a water-soluble peroxide radical initiator and then heated and dried to form water-insoluble crosslinked polymers. Initiator levels as low as 0.01 wt % based on polymer solids is claimed to effect the crosslinking, although 0.5% to 5% of initiator is normally used. However, it is noticed that very high temperatures, usually higher than 120° C., are used. At such high reaction temperatures, the added initiator may not be the only contribution to the achieved crosslinking. U.S. Pat. No. 3,168,500 reports that at temperatures of over about 120° C., polymers of acrylamide tend to crosslink by themselves without a radical initiator. The self-crosslinking of polyacrylamide is believed to involve a different reaction mechanism, reaction with other units at the amide group to evolve ammonia. In addition, the reaction system for making water absorbent polymers is an open system. Crosslinking occurs at high polymer solids (50 to 90%) when most of the water in the initial mixture has been evaporated, though the presence of water is also critical for effective crosslinking. The acrylic polymers used in European Patent 208,945 for crosslinking contain at least 70% by weight of acrylic acid monomeric units and 60 to 90% of carboxyl groups from acrylic acid in the form of an alkali metal salt. The patent is related to making water-insoluble anionic polyacrylates used as superabsorbents for water.
Published European Patent 600,592 discloses a method for preparing low molecular weight, crosslinked, water-soluble anionic polymers by radical crosslinking. The chain combination reaction is carried out in the presence of water-soluble radical initiators such as those used in U.S. Pat. No. 3,168,500 for nonionic acrylamide polymers. However, while U.S. Pat. No. 3,168,500 reported that the pH of the system during the process is not critical, European patent 600,592 found that the pH had a dramatic effect on the chain combination reaction. An optimal pH of 5.0 was found for anionic acrylate polymers in European Patent 600,592. Also, while U.S. Pat. No. 3,168,500 reported that no reaction would occur with less than about 5% of the initiator for polyacrylamide, EP 600,592 used as little as 2% of sodium persulfate to increase the molecular weight of polyacrylates. EP 600,592 teaches that the chain combination reaction for the acrylate polymer can be effected even in the presence of significant amount of polymerizable monomer. On the other hand, U.S. Pat. No. 3,168,500 states that the starting acrylamide polymer should be “substantially free” or have no more than 1-2% of monomer. In EP 600,592, the starting acrylate polymer solution was heated to a reaction temperature of 90° C. The desired amount of radical initiator was then added over a relatively short period time (15 to 30 minutes). The reaction temperature was maintained for an additional time, usually less than 2 hours, to use up the initiator added for crosslinking. Reaction temperature, pH, amount of added initiator, and reaction time mainly control the extent of crosslinking and molecular weight increase after addition of the initiator. Initiator feed time is not used to control extent of crosslinking. The patent is related to making low molecular weight crosslinked polyacrylates for detergent and cleaning applications.
There is evidence that strong cationic polyelectrolytes behave differently from weak polycations in responding to binding polyvalent count ions (Ghimici et al, Journal of Polymer Science: Part B, Vol. 35, page 2571, 1997), which can be present in systems using ionic water-soluble radical initiators such as salts of persulfuric acid, perphosphonic acid and percarbonic acid. Furthermore, crosslinking between the strong electrolyte polymeric radicals can be limited due to electrostatic repulsion. Ma and Zhu (Colloid Polym. Sci, 277:115-122 (1999) have demonstrated that polyDADMAC cannot undergo radical crosslinking by irradiation because the cationic charges repel each other. On the other hand, nonionic polyacrylamide can be readily crosslinked by irradiation. Difficulty in crosslinking polyDADMAC with organic peroxides was reported by Gu et al. (Journal of Applied Polymer Science, Volume 74, page 1412, (1999)). Treating polyDADMAC with a dialkyl peroxide in the melt (140 to 180° C.) only led to degradation of the polymer as evidened by a decrease in intrinsic viscosity.